Activated Sludge Biosolids Research Articles

Variations of moisture and organics in activated sludge during Fe0/S2O82− conditioning–horizontal electro-dewatering process

The feasibility of using Fe0/S2O82− conditioning (zero valence iron (ZVI)/persulfate (ps)) integrated with horizontal electro-dewatering (HED) to improve the dewaterability of activated sludge (AS) was evaluated. The removal and migration of free and bound water in the ZVI/ps–HED process were determined, along with the organic matter migration between the solid and liquid phases of AS biosolids. Results showed that the optimum conditioning dosages were determined as 0.35 g ZVI/g dry solids (DS) and 0.15 g ps/g DS on the basis of capillary suction time variation. The lowest final water content (WC) of 83.67% in AS occurred after the HED treatment at 40 V and 120 min as determined using the response surface methodology. Despite the disruption of AS flocs and microbial cells through SO4−· oxidation during the ZVI/ps conditioning, the particle size and mass fractal dimension of the AS flocs remained relatively stable as the zeta potential increased from −8.5 mV to −4.6 mV. The coagulation of Fe3+ from the oxidation of Fe2+ by ps contributed to the relatively stable condition, which favored the subsequent momentum of the electroosmotic flow. Moreover, the ZVI/ps conditioning decreased the bound water content (BWC, from 1.83 g/g DS to 1.38 g/g DS) and increased the free water content (FWC, from 60.27 g/g DS to 60.91 g/g DS), indicating a transformation from free water to bound water. After the ZVI/ps–HED process, the FWC and BWC were removed significantly with ratios of 96.32% and 79.78%, respectively. Along with water removal through the ZVI/ps–HED process, the organic matter in the supernatant/filtrate initially increased and further rose at the HED stage. The analysis of the extracellular polymeric substance (EPS) content showed that the proteins (PNs) and polysaccharides (PSs) at the liquid and solid phases decreased due to oxidative degradation. Furthermore, the ZVI/ps stage significantly reduced the PNs content of slime to 57.22%, the PSs content of EPS to 68.50%, and the PN-like substances in the slime and tightly bound EPS to 74.90% and 52.47%, respectively. In addition, the WC of AS correlated with the contents of PN and microbial by-product-like material in slime, as well as tryptophan in TB-EPS, and the low EPS content in AS indicated good dewaterability. Thus, ZVI/ps should be selected as a pretreatment prior to HED.

Novel electro-dewatering system for activated sludge biosolids in bench-scale, pilot-scale and industrial-scale applications

To optimize the economics of electro-dewatering system in a new multi-layer vertical electric field for activated sludge biosolids, different-sized experiments were carried out in this study, including bench-scale, pilot-scale and industrial-scale applications. A bench-scale system was established to determine the optimal parameters of the new structure, including the mechanical pressure level, dewatering extent, processing time and sludge cake thickness. Then, a pilot-scale automated electro-dewatering system was developed to investigate the cathode fouling problems during continuous operations. A complete industrial-scale electro-dewatering system was established with 80t/d of sludge dewatering capacity based on the above work. The operational stability, the service life of the core parts and the economics were evaluated during a long time operating. Under various optimized parameters and control modes, 69% of the moisture in mechanically dewatered sludge could be reduced within 7–8min. The average energy consumption was 0.123kWh/kgdewater at 41kgdewater/(m2h) of anode processing capacity and the anode side temperature was lower than 65°C, when the anode utilization efficiency was 88.2%. The industrial-scale tests with optimized parameters demonstrated considerable economic benefits.

Migration and distribution of water and organic matter for activated sludge during coupling magnetic conditioning–horizontal electro-dewatering (CM–HED)

Magnetic micro-particles (MMPs) and magnetic field (MF) were utilized as a coupling conditioning unit before dewatering activated sludge (AS) under a horizontal electric field. The removal and migration of free and bound water in AS during coupling magnetic conditioning–horizontal electro-dewatering (CM–HED) were determined. The organic matter migration between the solid and liquid phases of AS biosolids was also analyzed. Results show that MMPs dosage and MF intensity were determined as 0.15 g/g dry solids and 0.065 T for the best dewaterability, respectively. The optimum dewatering conditions for CM–HED with the final water content of 89.98% were 40 V and 120 min as determined using the response surface methodology. MMPs conditioning could induce a slight coagulation among AS flocs, increase the particle size from 85.9 μm to 92.3 μm and decrease mass fractal dimension from 2.18 to 2.07. The MMPs-conditioned AS also showed a network-like structure, banded cells with shrunk surfaces. CM–HED process effectively reduced the free water content (FWC) and bound water content (BWC) but increased the portion of BWC in AS. The corresponding removal ratios of bound water and free water were 52.89% and 95.86% at the anode side and 46.28% and 92.75% at the cathode side, respectively. The coupling magnetic conditioning led to the largest BWC reduction of 23.14% in CM–HED process, and most of this reduction approaching 92.83% occurred during magnetic micro-particle conditioning stage. Gravity field caused the largest sludge reduction of 87.45%. During HED stage, the removal ratio of free water in AS was 63.73% at the anode side and 36.54% at the cathode side, while it was 21.9% and 10.96% for bound water, respectively. Along with water removed by CM–HED process, the organic matter in supernatant/filtrate increased, and a sharp rise occurred during HED stage. Meanwhile, the extracellular polymeric substance (EPS) contents initially decreased at MMPs–MF conditioning stage and then increased at the HED stage. Lower EPS contents in AS indicated better dewaterability. The largest reduction in the EPS content of AS at MMPs–MF conditioning stage corresponded to the largest decline in BWC. Furthermore, the protein-like substances in the supernatant/filtrate increased from 30% to 50%, whereas the fulvic acid-like and humic acid-like substances decreased from 54% to 27% at the HED stage. MMPs significantly reduced the polysaccharide and protein contents in the slime–EPS by 90.26% and 99.25%, respectively. Approximately 92.35% of humic acid-like materials in tightly bound EPS were reduced by MMPs. Thus, coupling magnetic conditioning should be selected as pretreatment prior to HED.

Inactivation mechanisms of bacterial pathogen indicators during electro-dewatering of activated sludge biosolids

Electro-dewatering is an energy-efficient technology in which an electric field can increase the dryness of biosolids from secondary wastewater treatment from 15% w/w to 30–50% w/w. Here, we address bacterial pathogen indicators inactivation (total coliforms, Escherichia coli and aerobic endospores) during electro-dewatering, investigating the roles of electrochemically generated oxidants, extreme pH, and high temperature (from Joule heating). Our results demonstrate that temperature is the primary factor affecting total coliforms and E. coli inactivation. First, several electro-dewatering cycles were used to increase sludge temperature to about 100 °C after 6 min, during which time the average pH decreased from 7 to 3.6 after 10 min. Total coliforms and E. coli MPNs reached their detection limits after 6 min (with 4–5 logs of inactivation for total coliforms and 3–4 logs for E. coli). In contrast, aerobic endospores were not inactivated under these conditions; rather, their germination appeared to be stimulated by 6–8 min of electro-dewatering. Second, the dewatering cake was separated into four horizontal layers. After 8 min of electro-dewatering, the pH in the top layers decreased to 3, whereas the pH in the bottom layers increased to 8. Inactivation of total coliforms and E. coli in the sludge cake was similar in all layers, increasing with time, suggesting that oxidants and extreme pH are secondary inactivation factors. Finally, electrodes were cooled to maintain a temperature less than 34 °C. Although pH decreased significantly after 12 min of electro-dewatering, there was no significant bacterial pathogen indicator inactivation at low temperature.

Alkaline extraction of wastewater activated sludge biosolids

Activated sludge produced by wastewater treatment facilities are a sub-utilized by-product whose handling and disposal represent significant costs to these facilities. In this work, we introduced a simple and effective alkaline extraction technique that extracts up to 75% of the sludge’s organic matter into a liquor containing potentially useful organic material (proteins, carbohydrates, etc.). The results suggest that at pH 11 and above, cell lysis occurs, liberating substantial quantities of organic material into the alkaline solution. When compared to a cation exchange resin (CER) extraction developed for analytical purposes, the alkaline extraction recovered 3× more organic material. The alkaline extract was highly surface active, despite containing a relatively small fraction of lipids. At pH 12 and above the lipid fraction was enriched with C15–C16 fatty acid residues, likely associated with cell membrane phospholipids as suggested by nuclear magnetic resonance spectroscopy ( 31P NMR). Size exclusion chromatography studies show that the extract is enriched with biopolymers or assemblies of molecular weights in the order of tens of thousands of Daltons. Potential uses for the extract are discussed.

Estimating potential risks to terrestrial invertebrates and plants exposed to bisphenol A in soil amended with activated sludge biosolids

Bisphenol A (BPA) is a high production volume substance primarily used to produce polycarbonate plastic and epoxy resins. During manufacture and use, BPA may enter wastewater treatment plants. During treatment, BPA may become adsorbed to activated sludge biosolids, which may expose soil organisms to BPA if added to soil as an amendment. To evaluate potential risks to organisms that make up the base of the terrestrial food web (i.e., invertebrates and plants) in accordance with international regulatory practice, toxicity tests were conducted with potworms (Enchytraeids) and springtails (Collembolans) in artificial soil, and six plant types using natural soil. No-observed-effect concentrations (NOEC) for potworms and springtails were equal to or greater than 100 and equal to or greater than 500 mg/kg (dry wt), respectively. The lowest organic matter-normalized NOEC among all tests (dry shoot weight of tomatoes) was 37 mg/kg-dry weight. Dividing by an assessment factor of 10, a predicted-no-effect concentration in soil (PNEC(soil)) of 3.7 mg/kg-dry weight was calculated. Following international regulatory guidance, BPA concentrations in soil hypothetically amended with biosolids were calculated using published BPA concentrations in biosolids. The upper 95th percentile BPA biosolids concentration in North America is 14.2 mg/kg-dry weight, and in Europe is 95 mg/kg-dry weight. Based on recommended biosolids application rates, predicted BPA concentrations in soil (PEC(soil)) would be 0.021 mg/kg-dry weight for North America and 0.14 mg/kg-dry weight for Europe. Hazard quotients (ratio of PEC(soil) and PNEC(soil)) for BPA were all equal to or less than 0.04. This indicates that risks to representative invertebrates and plants at the base of the terrestrial food web are low if exposed to BPA in soil amended with activated sludge biosolids.

Hg(II) and Pb(II) adsorption on activated sludge biomass: Effective biosorption mechanism

This study focuses on the removal mechanisms effective in the sequestration of Pb(II) and Hg(II) on nonviable activated sludge biosolids at batch biosorption tests. The raw biosolids were supplied from a nearby municipal wastewater treatment plant activated sludge aeration unit. The biosolids were conditioned by residing the dried biosolids in NaOH solution for 15 min and subsequently autoclaving at 18 psi at 121 °C for 30 min. IR spectral analysis of the conditioned biosolids disclosed that significant binding sites in biosorption were carboxyl, amine and phosphoryl groups. The functional groups on the conditioned biosolids were then stabilized by chemical methods and thus contribution of each functional group to metal ion sequestration in biosorption was disclosed. The effective binding mechanism in sequestration was further analyzed by metal ion fractionation and leaching tests. The results demonstrated that ion exchange was the most effective removal mechanism contributing to sequestration of metal ions on the biosolids. Desorption tests demonstrated that hydrochloric acid is a powerful agent to leach the sequestered metal ions from the biosolids. Regenerations carried out with NaOH solution did not result in any remarkable capacity decrease in the subsequent biosorption cycles.